This invention relates generally to encoding and decoding systems and particularly to techniques for audio scrambling and unscrambling in pay television signal encoding systems.
So-called pay television systems, normally operated in connection with cable connected television subscription services, have grown in popularity and have given rise to a need for added security to restrict certain programming to those who have specifically subscribed and paid for it. Common techniques for television signal encoding include suppressing the horizontal sync pulses and randomly inverting the video information. The audio portion of the program may also be encoded by a number of well-known techniques.
A very old audio encoding scheme involves modulating the audio information with a small offset frequency to displace the entire audio information spectrum upwards in frequency. Doing this, as is well-known, destroys the harmonic relationship between the various frequencies in the audio information and contributes to a significant lack of intelligibility when the audio information is recovered. The larger the offset frequency, the greater the unintelligibility factor.
With present day sophisticated tecnhology, the simple audio encoding systems of the prior art are not secure in that they can be readily comprised. Consequently, there has been a great increase in the number of "pirate" decoders which offer the benefits of the encoded program material without remuneration to the owner. While the art of encoding has advanced to a very high state, secure systems are complex and expensive. Cable television system operators are reluctant to incorporate expensive systems because the cost must be passed on to their customers. Consequently, there is a need in the art for relatively secure audio encoding at low cost.